111 research outputs found
On the disintegrating action of meteoritic impacts
Action on planets and asteroids of crater forming meteorite
Relativistic Model of Detonation Transition from Neutron to Strange Matter
We study the conversion of neutron matter into strange matter as a detonation
wave. The detonation is assumed to originate from a central region in a
spherically symmetric background of neutrons with a varying radial density
distribution. We present self-similar solutions for the propagation of
detonation in static and collapsing backgrounds of neutron matter. The
solutions are obtained in the framework of general relativistic hydrodynamics,
and are relevant for the possible transition of neutron into strange stars.
Conditions for the formation of either bare or crusted strange stars are
discussed.Comment: 16 pages, 4 figures. Submitted to IJMP
Direct Visualization of Laser-Driven Focusing Shock Waves
Cylindrically or spherically focusing shock waves have been of keen interest
for the past several decades. In addition to fundamental study of materials
under extreme conditions, cavitation, and sonoluminescence, focusing shock
waves enable myriad applications including hypervelocity launchers, synthesis
of new materials, production of high-temperature and high-density plasma
fields, and a variety of medical therapies. Applications in controlled
thermonuclear fusion and in the study of the conditions reached in laser fusion
are also of current interest. Here we report on a method for direct real-time
visualization and measurement of laser-driven shock generation, propagation,
and 2D focusing in a sample. The 2D focusing of the shock front is the
consequence of spatial shaping of the laser shock generation pulse into a ring
pattern. A substantial increase of the pressure at the convergence of the
acoustic shock front is observed experimentally and simulated numerically.
Single-shot acquisitions using a streak camera reveal that at the convergence
of the shock wave in liquid water the supersonic speed reaches Mach 6,
corresponding to the multiple gigapascal pressure range 30 GPa
On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space
Nano and micro meter sized dust particles travelling through the heliosphere
at several hundreds of km/s have been repeatedly detected by interplanetary
spacecraft. When such fast moving dust particles hit a solid target in space,
an expanding plasma cloud is formed through the vaporisation and ionisation of
the dust particles itself and part of the target material at and near the
impact point. Immediately after the impact the small and dense cloud is
dominated by collisions and the expansion can be described by fluid equations.
However, once the cloud has reached micro-m dimensions, the plasma may turn
collisionless and a kinetic description is required to describe the subsequent
expansion. In this paper we explore the late and possibly collisionless
spherically symmetric unconstrained expansion of a single ionized ion-electron
plasma using N-body simulations. Given the strong uncertainties concerning the
early hydrodynamic expansion, we assume that at the time of the transition to
the collisionless regime the cloud density and temperature are spatially
uniform. We do also neglect the role of the ambient plasma. This is a
reasonable assumption as long as the cloud density is substantially higher than
the ambient plasma density. In the case of clouds generated by fast
interplanetary dust grains hitting a solid target some 10^7 electrons and ions
are liberated and the in vacuum approximation is acceptable up to meter order
cloud dimensions. ..
Self-similar imploding relativistic shock waves
Self-similar solutions to the problem of a strong imploding relativistic
shock wave are calculated. These solutions represent the relativistic
generalisation of the Newtonian Gouderley-Landau-Stanyukovich problem of a
strong imploding spherical shock wave converging to a centre. The solutions are
found assuming that the pre-shocked flow has a uniform density, and are
accurate for sufficiently large times after the formation of the shock wave.Comment: 22 pages, 4 figures. Minor corrections and a discussion of the
singular C_ characteristic added. Accepted for publication in Physics of
Fluid
The New White Flight
White charter school enclaves—defined as charter schools located in school districts that are thirty percent or less white, but that enroll a student body that is fifty percent or greater white— are emerging across the country. The emergence of white charter school enclaves is the result of a sobering and ugly truth: when given a choice, white parents as a collective tend to choose racially segregated, predominately white schools. Empirical research supports this claim. Empirical research also demonstrates that white parents as a collective will make that choice even when presented with the option of a more racially diverse school that is of good academic quality.
Despite the connection between collective white parental choice and school segregation, greater choice continues to be injected into the school assignment process. School choice assignment policies, particularly charter schools, are proliferating at a substantial rate. As a result, parental choice rather than systemic design is creating new patterns of racial segregation and inequality in public schools. Yet the Supreme Court’s school desegregation jurisprudence insulates racial segregation in schools ostensibly caused by parental choice rather than systemic design from regulation. Consequently, the new patterns of racial segregation in public schools caused by collective white parental choice largely escapes regulation by courts.
This article argues that the time has come to reconsider the legal and normative viability of regulating racial segregation in public schools caused by collective white parental choice. The article makes two important contributions to the legal literature on school desegregation. First, using white charter school enclaves as an example, it documents the ways in which school choice policies are being used to allow whites as a collective to satisfy their preference for segregated predominately white schools. Second, the article sets forth both constitutional and normative arguments for regulating the private choices that result in stark racial segregation patterns in public schools
Deep Impact Mission to Tempel 1 Favours New Explosive Cosmogony of Comets
The assumption that short-period (SP) comets are fragments of massive icy
envelopes of Ganymede-like bodies saturated by products of ice electrolysis
that underwent global explosions provides a plausible explanation of all known
manifestations of comets, including the jet character of outflows, the presence
of ions in the vicinity of the nucleus, the bursts and splitting of cometary
nuclei, etc., with solar radiation initiating burning of the products of
electrolysis in the nucleus. As shown persuasively by numerical simulation
carried out in hydrodynamic approximation, the shock wave initiated by the Deep
Impact (DI) impactor in the cometary ice saturated originally by the
electrolysis products 2H2 + O2 is capable of activating under certain
conditions exothermal reactions (of the type O2 + H2 + organics = H2O + CO +
HCN + other products of incomplete burning of organics including its light and
heavy pyrolyzed compounds, soot, etc.), which will slow down shock wave damping
(forced detonation) and increase many times the energy release. As a result,
the measured energetics of ejections and outflows from the crater have to
exceed the DI energetics. Analysis of different clusters of the DI experiment
data confirms these conclusions and expectations and thus it favours the
planetary origin of comets.Comment: 21 pages incluging 3 figure
Perfect Fluid Theory and its Extensions
We review the canonical theory for perfect fluids, in Eulerian and Lagrangian
formulations. The theory is related to a description of extended structures in
higher dimensions. Internal symmetry and supersymmetry degrees of freedom are
incorporated. Additional miscellaneous subjects that are covered include
physical topics concerning quantization, as well as mathematical issues of
volume preserving diffeomorphisms and representations of Chern-Simons terms (=
vortex or magnetic helicity).Comment: 3 figure
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
- …